This disclosure is directed to the problem of cleaning a nest of heat exchanger tubes. Heat exchangers are normally formed in a nest or bundle of many straight parallel pipes or tubes. They typically are not very long and not very large. They are used in transferring heat from a heated fluid to another fluid, and most often are found in petrochemical complexes. It is not uncommon to install a heat exchanger with a furnace to heat water so that steam can be formed. In heating water, minerals in the water plate on the inside surfaces of the heat exchanger tubes. This forms various carbonate deposits. Even where the water is purified significantly in advance, some carbonates and other deposits are formed. To the extent that other minerals are found in the feed, different deposits may be placed in the inside wall but they accumulate and form a serious problem. The same problem arises where petroleum fractions in fluids are heated. Heavier molecules will plate or coat the inside surface of the tubes. Over time with the constant exposure to heat, the materials can become relatively hard. On the scale of hardness, they can exceed that of typical carbonates. For instance, where the fluid heated is water, the water will deposit minerals to form a deposit which has the hardness approximately of sheet rock or comparable materials. If it becomes even harder over time, hence more coking, it can become as hard as concrete. On the scale of hardness using diamond as a 10.0 maximum value, it is not uncommon to build heat exchanger tube deposits which have a hardness of 5 to 7 mohs. This creates a serious problem because it reduces the capacity of the heat exchanger tubes. With a tube having a radius of precisely 1.000 inches. A deposit of 0.01 inches does not seem like an overwhelming problem. When the deposit becomes 0.02 inches, it still seems relatively trivial, that nevertheless represent a 4% loss in effective cross-sectional area. With a deposit twice as thick, the loss is 12%. As one will see, this is a substantial reduction in flow cross-sectional area.
Tube cleaning in a heat exchanger is periodically required. There is however a balance which can be determined for the frequency of tube cleaning. Tube cleaning normally involves letting the deposits build to such a point that the loss of efficiency is substantial. In a heat exchanger, the loss of efficiency may not be solely from the loss of cross-sectional area suggested by the arithmetic noted above. Rather, the problem may be compounded by the insulative nature of the deposits. Especially when they begin coking, they become a hard ceramic which is slightly porous. It may well serve as a thermal barrier in the heat exchanger tubes and reduce the effective heat transfer rate through the wall between the two fluids. As the heat transfer becomes less efficient, greater heat levels are required. This may harden the deposits to an even greater level. In that instance, the BTU consumption rate for the furnace or boiler is much higher so the furnace or boiler is then taken out of service, the tubes are cleaned and it is restored to service. In effect, the cost of utilities increase steadily with accumulated deposits, and can become sufficient that cleaning is mandated at a cost of interrupting the process.
A plant shutdown is an expensive undertaking. It is not so much the cost of the cleaning service that is costly; the problem derives from the loss of production. It is not uncommon for a single boiler to require cleaning yet the plant itself may produce a revenue stream of several million dollars per day. When the plant is out of service for two or three days for repairs, the cost savings in reduced utilities may seem great, but it is relatively minor in comparison with the loss of production. By balancing this, plant shutdowns are timely implemented.
Consider now the benefits of cleaning the heat exchanger tubes. Typically they are small, up to about 5 or 6 inches in maximum diameter. Most heat exchanger tubes are in the range of about 1 or 2 inches. Assume that the deposits have been hardened with temperature to about 5 or 6 mohs hardness over time. The present disclosure sets forth a cleaning device which can be used at elevated temperatures. The exterior of the heat exchanger may be exposed to the updraft of a furnace. The interior of the tubes is exposed to the steam, heated water or other fluid receiving the heat transfer. In that region, it is possible to install pig launching apparatus which will position a pig in line for transfer through a tube of the furnace heat exchanger. The heat exchanger in that region however is operated at an elevated temperature. The actual temperature depends on the nature of the process, but using conversion of water into steam as an example, the system is able to clean the tubes dynamically during operation. For specified temperature ranges, typically up to about 500.degree. F., the present apparatus sets forth a pig for dynamic cleaning. The furnace and heat exchanger have to be taken out of service to enable cleaning when the materials of the equipment cannot tolerate the elevated temperatures. The present disclosure sets out a polyurethane based multiple component pig assembly which is anchored on a central bolt serving as a mounting mandrel.
It is possible to make a cast polyurethane or other elastomer body having metal studs extending from the side which scratch, scrape or otherwise break up deposits, especially those that have been converted into coke like substances and at 5 or 6 on the hardness scale. It is however difficult to get dimensional stability and a firm anchor by attachment of a bolt in a polyurethane foam body. The dimensional stability is in part dependent on the density of the foam. If the foam is light weight, there is an inherent lack of structural stability. Light weight foam is preferred because it reduces the cost of the pig. A heavy foam is not desirable because the cost goes up, and yet dimensional stability is improved only slightly. If a stud is mounted entirely through a pig body, it can be stable and position the tip precisely but that requires a relatively large diameter body to anchor the stud. For instance, a pig of 12 to 24 inches diameter will support and accurately position a set of studs. In that arrangement, the studs are mounted so that the tip cuts to the right depth and is controlled in relative location. Pigs of that size cannot be used in cleaning small diameter tubes in heat exchangers. The present disclosure is directed to a pig which uses an insertable steel disk. The steel disk is carefully drilled at selected locations, and threaded bolts are anchored around the periphery thereby defining a carefully positioned set of studs for contact against the wall of the heat exchanger. Precise positioning is relatively important. If the pig is placed in a heat exchanger tube having an ID of 4.000 inches, it is desirable that the stud head protrude to that length but not further. Precise positioning is necessary to break up the scale. If for instance a stud does not penetrate the scale, then there will be no scale removal. On the other hand, if the stud is too long, it may gouge and scar the thin wall heat exchanger tube. Positioning of that precision requires lateral support for each stud so they do not wobble or bind.
The present disclosure is directed to a multiple part pig for heat exchangers. It is constructed so that the pig body assembles and disassembles. The pig is assembled so that it supports a selected number of flat steel disks of specified thickness. Around the outside, they have a number of calculated openings which mount a set of threaded studs. The drilled and tapped openings enable positioning of the studs at a selected cylindrical position around the centerline axis, thereby sharply cutting the scale in a heat exchanger without damaging the heat exchanger. This enables a set of disks to be interchanged thereby putting longer studs on the pig for wiping the tubing. This enables progressive breakage of the scale deposits on the interior of the heat exchanger tubes. The pig operates bidirectionally and is symmetrical comparing the two ends. The pig construction utilizes a skin covered polyurethane body. The body parts are coated at the surface or skin to assure that the components are able to exclude penetration or absorption of organic materials.